There is an existing thread on the Westin Xian:
Westin Xian, China [Master Thread]
And the Sheraton Xian:
Sheraton Xian (Xi'an), China [Master Thread]
There is a stay review for the Sheraton Xian North City:
Sheraton Xian North City Hotel, China [Master Thread]
It looks like there are other Marriott Bonvoy hotels in Xian, but no stay posts yet in those hotel threads.